In conventional vacuum cleaners, an airflow pathway comprises an inlet for dirty air, which is provided for example as part of a floor-cleaning head, an outlet for clean air and a source of suction power. The source of suction power, which is typically a motor and a fan driven by the motor, generates a pressure differential between the dirty air inlet and the clean air outlet, which draws air in through the inlet and expels the air through the outlet. A filter or other separation device such as a cyclone located in fluid communication between the inlet and the outlet separates out dust and dirt from the air as it passes along the airflow pathway. In conventional vacuum cleaners, the size of the pressure differential and therefore the strength of suction generated at the dirty air inlet is usually modulated in one of two ways, as follows.
Firstly, a second inlet for clean air may be provided to the airflow pathway which can be opened and closed by a user as desired. This second inlet typically takes the form of a bleed valve provided on a wand of the vacuum cleaner having the inlet for dirty air located at one end thereof. Thus as the bleed valve is opened by the user, air is drawn into the airflow pathway by the source of suction power through both the inlet for dirty air and the second, clean air inlet. Since the strength of the source of suction power itself has not changed, the total rate of air movement (i.e. volume of air moved per unit time) through the vacuum cleaner does not change either. Accordingly, the volume of air entering the dirty air inlet per unit time drops in order to accommodate the increased volume of air also entering the airflow pathway through the second, clean air inlet. This first technique for modulating the size of the pressure differential between the dirty air inlet and the clean air outlet has the advantage that it is cheap and simple to manufacture. However, it also has the disadvantage that it gives little control to the user beyond two settings in which the bleed valve is either open or closed. In order to alleviate this problem somewhat, a more sophisticated version of the bleed valve may also allow the size of the second, clean air inlet to be varied but this gives little precise control to the user.
In a second technique for modulating the size of the pressure differential between the dirty air inlet and the clean air outlet, the source of suction power is instead provided with a mechanism for adjusting the amount of power supplied to the motor. Accordingly, the strength of the source of suction power is itself changed and the total rate of air movement through the vacuum cleaner changes with it. Thus as the motor power adjustment mechanism is operated by a user, the volume of air entering the dirty air inlet per unit time varies in proportion to the amount of power supplied to the motor. The mechanism for adjusting the amount of power supplied to the motor typically takes the form of a rheostat or a switch having several different power settings which the user may select, and may also incorporate some control electronics as well. This second technique has the advantage that the mechanism allows for sophisticated and precise control of the volume of air entering the dirty air inlet by the user, but it also has the disadvantage that it is more expensive and difficult to manufacture than a bleed valve. Because of its greater degree of complication, it is also more liable to malfunction than a bleed valve.